The present invention relates to a structure and process for injection molding optical and compact disks.
Injection molding involves injecting molten thermoplastic resin into a mold apparatus. Molds for injection molding of thermoplastic resin are usually made from metal materials such as iron, steel, stainless steel, aluminum, or brass because these materials have high thermal conductivity and thus permit the melt of thermoplastic resin to cool rapidly and shorten the molding cycle time. A drawback to rapid cooling in these molds is that the injected resin freezes instantaneously at the mold surface, resulting in a thin solid layer. Quick quenching of the melt at the mold surface creates a rough surface (instead of a high quality optical surface) which can impact disc performance. The quick solidification of the melt combined with variable radial flowability of the materials makes the uniform melt flow and uniform surface replication required for an optical disk difficult to achieve. Non-uniform flow and surface imperfections can result in areas on an optical disk with high bit errors.
In the injection molding of compact discs, for audio, video, or computer data storage and retrieval applications, heat transfer through the mold thus has a strong effect on molding time and disc attributes such as birefringence, flatness, and accuracy of feature replication. For a process to be economical, a careful balance must be maintained between low cycle times and the process parameters needed to meet the quality requirements.
A method for affecting heat transfer and improving the cycle time during injection molding by incorporating insulation into the mold has been described in commonly assigned Kim et al., U.S. Pat. No. 5,458,818. In Kim et al., a multilayer mold is used in which a metal core has an insulating layer bonded thereto for slowing the initial cooling of the resin during the molding operation. The insulating layer comprises material having both low thermal diffusivity and conductivity, thus slowing the cooling of the molded resin, and good resistance to high temperature degradation, permitting use in a mold maintained at high temperatures. One or more skin layers of hard material, typically metal, can be bonded to the insulating layer.
Another method for affecting heat transfer is described in Nakamura et al., Japanese Unexamined Patent Application Disclosure Bulletin No. 88-71325. In Nakamura et al., a layer of synthetic resin is formed on a stamper by coating or lamination before the stamper is placed on the core molding surface of the metal mold.
The use of an insulating layer is desirable so as to cause a minimal change in the size and shape of the molding tool and equipment. For compact discs, stringent requirements of optical clarity, surface morphology, and replication of surface features of sub-micron dimensions obviate the use of common insulating materials, which do not provide a smooth enough surface, are not stable for long periods at the mold temperature, or cannot withstand the repeated application of high pressure during the molding process.
For a sheet or film to be useful for managing heat transfer for a mold it must have a very smooth surface ( less than 0.1 xcexcm surface roughness) over a large area so that it will not introduce feature replication errors or surface imperfections into the manufactured disk. It is also preferred that the surface be compliant to attenuate minor imperfections in the molding tool while maintaining mechanical and dimensional integrity during the molding process.
It is therefore seen to be desirable to provide a structure and method for molding optical disks having improved surface replication and improved molding characteristics.
Briefly, in accordance with one embodiment of the present invention, a method for molding an optical disk comprises: applying a thermally insulative insert coating to at least one thermally insulative mold insert to provide at least one coated mold insert having a reduced surface roughness; positioning the at least one coated mold insert between a thermally conductive mold form and a portion of a thermally conductive mold apparatus; injecting a molten thermoplastic material into the mold apparatus; retaining the material in the mold apparatus for a time sufficient for the molten thermoplastic material to cool below its glass transition temperature to form the optical disk; and ejecting the optical disk from the mold apparatus.
In a related embodiment, the thermally insulative mold insert is coated on the thermally conductive mold form with the mold insert having a coefficient of thermal expansion compatible with the coefficient of thermal expansion of the mold form. An adhesion promoter can be applied to the mold form prior to the coating of the mold insert.
In another related embodiment, the thermally insulative mold insert is laminated on the thermally conductive mold form using an adhesive with the mold insert having a coefficient of thermal expansion compatible with the coefficient of thermal expansion of the mold form and the adhesive comprising a material which does not significantly shrink and which has a coefficient of thermal expansion compatible with the coefficients of thermal expansion of the mold form and the mold insert.
According to another embodiment of the present invention, a mold insert for being positioned in a mold apparatus between the mold apparatus and a mold form comprises a layer of thermally insulative mold insert material and an insert coating applied on at least one surface of the mold insert material for providing a reduced surface roughness.
In a related embodiment, a mold form and a mold insert for being positioned in a mold apparatus comprise an adhesion promoter overlying a thermally conductive mold form and a thermally insulative mold insert coated on the adhesion promoter and the thermally conductive mold form, the mold insert having a coefficient of thermal expansion compatible with the coefficient of thermal expansion of the mold form.
In another related embodiment, a mold form and a mold insert for being positioned in a mold apparatus comprise: an adhesive between a thermally conductive mold form and a thermally insulative mold insert, the mold insert having a coefficient of thermal expansion compatible with the coefficient of thermal expansion of the mold form, the adhesive comprising a material which does not significantly shrink and which has a coefficient of thermal expansion compatible with the coefficients of thermal expansion of the mold form and the mold insert.
According to another embodiment of the present invention, an optical disk mold apparatus comprises: at least one coated mold insert comprising a thermally insulative insert coating applied to at least one thermally insulative mold insert to provide at least one coated mold insert having a reduced surface roughness; a thermally conductive mold form; and a thermally conductive mold apparatus. The at least one coated mold insert being positioned between the thermally conductive mold form and a portion of the thermally conductive mold apparatus.
In a related embodiment, the optical disk mold apparatus includes an adhesion promoter or an adhesive overlying a thermally conductive mold form and a thermally insulative mold insert coated on the adhesion promoter or adhesive and the thermally conductive mold form, the mold insert having a coefficient of thermal expansion compatible with the coefficient of thermal expansion of the mold form.
According to another embodiment of the present invention, a method for fabricating a mold insert for use in molding an optical disk comprises: applying a release layer to a substrate; coating a solution of liquid insulative mold insert material on the release layer; curing the mold insert material to form the mold insert; and removing the mold insert from the release layer and the substrate.